1. Technical Field
This disclosure relates to digital mask-image-projection-based additive manufacturing.
2. Description of Related Art
Layer-based additive manufacturing (AM) manufactures solid objects by the sequential delivery of energy and/or material to specified points in space to produce those solid objects. Multiple materials or functionally graded material may be added in a single component during the building process. An example of such multi-material AM systems is the OBJET Connex series of 3D printers. Based on its PolyJet Matrix Technology, these three-dimensional 3D printers may be capable of manufacturing complex internal structures with digital materials. That is, by combining two base materials in specific concentrations and structures, as many as 51 different materials can be created in a single printed part. Hence, product components can have material designs with desired mechanical properties. For example, both soft and hard materials can be embedded in products such as tooth brushes and remote controllers.
Recent 3D printer development has included digital material fabrication in which two base materials are used to define a wide variety of new materials. The OBJET Connex machines jet model materials from designated micro-scale inkjet printing nozzles. Such a process can have inherent limitations on the selection of base materials, since the jetted liquid may need to have certain viscosity and curing temperature properties in order to be jetted.
Besides inkjet printing technology, a fused deposition modeling (FDM) process can be extended for fabricating parts out of multi-materials, since FDM already has separate extrusion nozzles for the build and support materials. Khalil et al., Khalil, S, Nam, J, Sun, W (2005), Multi-nozzle deposition for construction of 3D biopolymer tissue scaffolds, Rapid Prototyping Journal, Vol. 11 (1), pp. 9-17 presented a multi-nozzle deposition system for producing 3D tissue-engineered scaffolds. However, the FDM process may have limitations on its minimum nozzle size and may be relatively slow. Hence, FDM may not be suitable for digital material fabrication.
There have been attempts at using selective laser sintering (SLS) for multi-material fabrication. See Jackson, B, Wood, K, Beaman, J J (2000), Discrete multi-material selective laser sintering: development for an application in complex sand casting core arrays, In: Proceedings of Solid Freeform Fabrication Symposium, The University of Texas at Austin, Austin, Tex., pp. 176-182; Liew, C L, Leong, K F, Chua, C K, Du, Z (2001), Dual material rapid prototyping tech-niques for the development of biomedical devices, Part I, Space creation, Int. J. Adv. Manuf. Technol., Vol. 18 (10), pp. 717-723; Liew, C L, Leong, K F, Chua, C. K, Du, Z (2002), Dual material rapid prototyping techniques for the development of biomedical devices, Part II, Secondary powder deposition, Int. J. Adv. Manuf. Technol. Vol. 19 (9), pp. 679-687; Santosa, J, Jing, D, Das, S (2002), Experimental and numerical study on the flow of fine powders from small-scale hoppers applied to SLS multi-material deposition, In: Proceedings of Solid Freeform Fabrication Symposium, The University of Texas at Austin, Austin, Tex., pp. 620-627; Regenfuss, P, Streek, A, Hartwig, L, Klötzer, S, Brabant, Th, Horn, M, Ebert, R, Exner, H (2007), Principles of laser micro sintering, Rapid Prototyping Journal. Vol. 13 (4), pp. 204-212. However, accurate material feeding and recoating may be required by the digital material fabrication, but may be difficult to integrate into an SLS process.
Another AM process is Stereolithography Apparatus (SLA). By using a laser and liquid photocurable resin, SLA may provide a high quality surface finish, dimensional accuracy, and a variety of material options. To address its speed limitation, a mask-image-projection-based Stereolithography (MIP-SL) process may be considered instead. An illustration of an example MIP-SL process and results that it can achieve are shown in FIGS. 1A and 1B. Instead of the laser used in SLA, a Digital Micromirror Device (DMD) may be used in the MIP-SL process to dynamically define mask images to be projected on a resin surface area. A DMD is a micro-electromechanical system (MEMS) device that enables one to simultaneously control a large number of small mirrors (e.g., about 1 million) to turn on or off a pixel each at over 5 KHz. In the MIP-SL process, a three-dimensional (3D) CAD model of an object may first be sliced by a set of stacked, horizontal planes. Each thin slice may then be converted into a two-dimensional (2D) mask image. The planned mask image may then be sent to the DMD. Accordingly, the image may be projected onto a resin surface, such that liquid photo curable resin can be selectively cured to form a layer of the object. By repeating the process, 3D objects can be formed on a layer-by-layer basis. Compared to the laser-based SLA, the MIP-SL process can be much faster due to its capability to simultaneously form the shape of a whole layer. Two test parts built by a prototype MIP-SL system using two different materials are also shown in the FIG. 1.
Principles of a Mask-Image-Projection-Based SL System
Multi-Material Fabrication Limitation
Multiple vats may be required for different types of liquid resin in the multi-material SLA and MIP-SL processes. As a natural extension to the single material SLA system, Maruo, S, Ikuta, K, and Ninagawa, T (2001), Multi-polymer microstereolithography for hybrid opto-MEMS, Proceedings of the 14th IEEE International Conference on Micro Electro Mechanical Systems (MEMS 2001), pp. 151-154, presented a multiple material stereo lithography system by manually removing the vat from the platform and draining the current material, rinsing the vat, returning the vat to the platform, and dispensing a prescribed volume of a different material into the vat. However, based on the lengthy process of manually changing the materials, the system may be limited to simple 2.5D microstructures. Wicker, R, Medina, F, and Elkins, C (2005), Multiplematerialmicro-fabrication: extending stereo lithography to tissue engineering and other novel application, In: Proceedings of Annual Solid Freeform fabrication Symposium, Austin, Tex., pp. 754-764; Choi, J, Kim, E H, and Wicker, R (2011), Multiple-material stereolithography, Journal of Materials Processing Technology, Vol. 211/3, pp. 318-328; Wicker, R, Medina, F, and Elkins, C, (2009), Multi-material stereolithography, U.S. Pat. No. 7,556,490, extended the work by developing a multiple vat carousel system to automate the building process including washing, curing and drying cycle between build materials. Based on similar ideas, Choi, J W, MacDonald, E, Wicker, R (2010), Multi-material microstereolithography, Int. J. Adv. Manuf. Technol. Vol. 49, pp. 543-551, reported a multi-material MIP-SL system for fabricating micro-scale objects. Arcaute, K, Zuverza, N, Mann, B, and Wicker, R (2006), Development of an automated multiple material stereolithography machine, In: Proceedings of Annual Solid Freeform Fabrication Symposium, Austin, Tex., pp. 624-635; Han, L, Suri, S, Schmidt, C E, and Chen, S (2010), Fabrication of three-dimensional scaffolds for heterogeneous tissue engineering, Biomed Microdevices, No. 12, 721-725, also presented an automatic material switching approach by dispensing the solution using a pipette into a custom-made small vat, and subsequently washing the current solution before changing to the next solution. Based on the technique, fabricated 3D scaffolds for heterogeneous tissue engineering have been demonstrated.
A challenge in the use of multiple materials in SL may be how to manage material contamination between changing different materials used in the fabrication process. The previous research of Maruo, S, Ikuta, K, and Ninagawa, T (2001), Multi-polymer microstereolithography for hybrid opto-MEMS, Proceedings of the 14th IEEE International Conference on Micro Electro Mechanical Systems (MEMS 2001), pp. 151-154; Wicker, R, Medina, F, and Elkins, C (2005), Multiplematerialmicro-fabrication: extending stereo lithography to tissue engineering and other novel application, In: Proceedings of Annual Solid Freeform fabrication Symposium, Austin, Tex., pp. 754-764; Choi, J, Kim, E H, and Wicker, R (2011), Multiple-material stereolithography, Journal of Materials Processing Technology, Vol. 211/3, pp. 318-328; Wicker, R., Medina F, and Elkins, C (2009), Multi-material stereolithography, U.S. Pat. No. 7,556,490; Choi, J W, MacDonald, E, Wicker, R (2010), Multi-material microstereolithography, Int. J. Adv. Manuf. Technol, Vol. 49, pp. 543-551; Arcaute, K, Zuverza, N, Mann, B, and Wicker, R (2006), Developmentof an automated multiple material stereolithography machine, In: Proceedings of Annual Solid Freeform Fabrication Symposium, Austin, Tex., pp. 624-635; Han, L, Suri, S, Schmidt, C E, and Chen, S (2010), Fabrication of three-dimensional scaffolds for heterogeneous tissue engineering, Biomed Microdevices, No. 12, 721-725, on developing multi-material SLA and MIP-SL systems are all based on top-down projection. As shown in FIG. 2, to accommodate part size in the Z direction, a large tank may have to be maintained for keeping the resin level. Due to the deep vat, draining and cleaning the current resin before changing to another resin vat may take a long time and may lead to significant material waste. To address the problem, Kim, H, Choi, J, and Wicker, R (2010), Process planning and scheduling for multiple material stereolithography, Rapid Prototyping J. Vol. 16, No. 4, pp. 232-240; Kim, H, Choi, J, MacDonald, E, and Wicker, R (2010), Slice overlap detection algorithm for the process planning of multiple material stereolithography apparatus, Int. J. Adv. Manuf. Technol. Vol. 46, No. 9, pp. 1161-1170, presented a process planning approach to minimize the material changeover number for a given multi-material CAD model. That is, if different materials are separated in a CAD model, one material can be built fully, or as much as possible, before transferring to another material. Even though the approach may be able to reduce the material changeover efforts, it may not be a general approach, especially for digital material fabrication in which different materials may be interlocked with each other.
Speed Limitation of Single-Material Fabrication
In the MIP-SL process, the building time of each layer may consist of spreading liquid resin into a uniform thin layer and curing the formed liquid layer into a solid layer. Compared to a laser beam that is used in the SLA process, the DMD used in the MIP-SLA process can dramatically decrease the curing time of a layer. Hence, the bottleneck for achieving a fast building speed may be the spreading of liquid resin into uniform thin layers.
Research systems Chatwin C, M Farsari, S Huang, M Heywood, P Birch, R Young, J Richardson (1998), UV micro-stereolithography system that uses spatial light modulator technology, Applied Optics, Vol. 37, pp. 7514-22; Bertsch, A, P Bernhard, C Vogt, P Renaud (2000), Rapid prototyping of small size objects, Rapid Prototyping Journal, Vol. 6, Number 4, pp. 259-266; Stampfl, J, H Fouad, S Seidler, R Liska, F Schwager, A Woesz, P Fratzl (2004), Fabrication and moulding of cellular materials by rapid prototyping, Int. J. Materials and Product Technology, Vol. 21, No. 4, pp 285-296; Sun C, N Fang, D Wu, X Zhang, (2005), Projection micro-stereolithography using digital micro-mirror dynamic mask, Sensors and Actuators A. Vol. 121, pp. 113-120; Lu, Y, G Mapili, G Suhali, S C Chen, K Roy, (2006), A digital micro-mirror device (DMD)-based system for the micro fabrication of complex, spatially patterned tissue engineering scaffolds, Journal of Biomedical Materials Research A, Vol. 77A (2), pp. 396-405; Limaye, A, D W Rosen, (2007), Process planning method for mask projection micro-stereolithography, Rapid Prototyping Journal, Vol. 13, No. 2, pp. 76-84; Choi, J, R B Wicker, S Cho, C Ha, and S Lee, (2009), Cure depth control for complex 3D microstructure fabrication in dynamic mask projection microstereolithography, Rapid Prototyping Journal, Vol. 15 (1), pp. 59-70 and commercial systems, such as EnvisionTEC and V-Flash desktop modeler, have been developed based on the mask image projection approach. Most of the developed systems are based on the top-down projection as shown in FIG. 1. Suppose dLT is the layer thickness. After a previous layer has been cured, the platform in such a system usually moves down a certain distance d and then up by d-dLT in order to spread liquid resin into a uniform thin layer. In addition to the Z movement, a recoating process may be required to sweep through the platform such that the top surface can be flattened. For resin with low viscosity, a deep-dip recoating approach has also been developed to replace the surface sweeping approach. After the up and down movements in the Zaxis, a sufficient waiting time may be required for the liquid resin to settle down into a flat surface. However, such recoating methods may take over a minute, which may limit the building speed of the MIP-SL process. Consequently, the building time of such MIP-SL systems may still be measured in hours.
Part Separation of Traditional Bottom-Up Projection Based MIP-SL Process
In the bottom-up projection based MIP-SL process, a cured layer may be sandwiched between the previous layer and the resin vat. The solidified material may adhere strongly to the corresponding rigid or semi-rigid transparent solidification substrate, causing the object to break or deform when the build platform and vat are pulled apart during the building process.
One approach to conquer the attachment force may be to increase the exposure to significantly over-cure the current layer such that its bonding force with the previous layer can be increased. However, over-curing can lead to poor surface quality and inaccurate dimensions. Another approach to address the problem may be to apply a certain coating on the resin vat such that the attachment force can be reduced. Suitable coatings, including Teflon and silicone films, can help the separation of the part from the vat. See Chen. Y, Zhou, C, and Lao, J (2011), A layerless additive manufacturing process based on CNC accumulation, Rapid Prototyping Journal, Vol. 17, No. 3, pp. 218-227; Huang, Y M, Jiang, C P (2005), On-line force monitoring of platform ascending rapid prototyping system, Journal of Materials Processing Technology, Vol. 159 pp. 257-64. A coated Teflon glass has also been used in the machines of Denken, SLP-4000 Solid Laser Diode Plotter, Product Brochure, Denken Corporation, Japan, 1997; and the EnvisionTEC ULTRA.
Even with an intermediate material, the separation force can still be relatively large. Huang, Y M, Jiang, C P (2005), On-line force monitoring of platform ascending rapid prototyping system, Journal of Materials Processing Technology, Vol. 159 pp. 257-64, investigated the attachment force for the coating of an elastic silicone film. Based on a developed on-line force monitoring system, test results indicate that the pulling force increases linearly with the size of the working area. Experiments indicate that, for a square of 60×60 mm, the pulling force to separate the part from the film can be greater than 60 N. Such a large attachment force between the cured layer and the vat can be a key challenge in the development of the bottom-up projection based MIP-SL system.
Another type of coating material, Polydimethylsiloxane (PDMS, Sylgard 184, Dow Corning), may be applied on the resin vat. This selection is based on a property of the PDMS film during the polymerization process that was identified in Dendukuri, D, Pregibon, D C, Collins, J, Hatton, T A, and Doyle, P S (2006), Continuous-flow lithography for high-throughput microparticle synthesis, Nature Mater., Vol. 5, pp. 365-369, who presented a photolithography-based microfluidic technique for continuously fabricating polymeric particles. The developed technique is based on the oxygen-aided inhibition near the PDMS surfaces to form chain-terminating peroxide radicals. In the process, a very thin oxygen inhibition layer (˜2.5 μm) is formed that can prevent the cured layer from attaching to the PDMS film.
Separation Forces for Solidified Resin Based on the PDMS Film
A set of physical experiments have been performed to investigate the separation force of a cured layer based on a coated PDMS glass. The setup for measuring the pulling force is shown in FIG. 4A. Two Flexi Force sensors (Tekscan, South Boston, Mass.) with a range of 0-25 lbs are sandwiched between the fixture and vat. Since the vat is free at the bottom and the side, and only fixed at the top, the pulling force by the part will be transferred to the sensors when the platform rises. The two sensors are connected to a microcontroller, which can sample and record the sensors' readouts at over 3 KHz. In the experiments, a given mask image was used to build a certain number of layers (e.g. 25 layers). The separation force in the building process of the next few layers was then recorded. For each layer, after the designed mask image has been exposed for a certain time, the platform is raised up slowly at 0.6 mm/sec and the related readouts of the two sensors are then recorded.
Three factors potentially affecting the separation force include: (1) exposure time; (2) image area; and (3) image shape. To understand the effects of these factors, designed experiments were conducted. FIG. 4B shows a set of mask images that have been used in the experiments for testing the effect of image shapes. The tested projection patterns, including triangle, square, hexagon, circle, t-shape, u-shape, band, and star-shape, have the same area in each test. FIG. 5 shows the measured separation forces of a sensor for different test cases. The horizontal axis indicates the distance in the Z direction (in the unit of 10 μm), and the vertical axis indicates the recorded pulling force (in ounces).
Based on the experimental results, it can be observed that:
(1) As the Z stage moves up, the separation force increases until it reaches a peak value when the cured layer is detached from the PDMS film;
(2) The peak force gets larger when the same mask image is exposed longer;
(3) The peak force gets larger when a larger image area is projected;
(4) The image shape has more complex effects on the peak force. In addition, their effects may interact with the exposure time and the projection area;
(5) With the coated PDMS film on the vat, the separation force may still be considerably large (˜100 oz or 27.8 N for an image area of 625 mm2 with 1 second exposure).
Separation Force for Liquid Resin without Curing
A similar set of experiments was conducted to analyze the pulling force of a part without liquid resin being cured between the part and the vat. In the experiments, an image of a square (35 mm×35 mm) was used to build a certain number of layers (e.g. 25 layers). The built part was then lowered to form a certain gap with the PDMS film. Without exposing any image to cure liquid resin, the platform was then raised up slowly at 1.2 mm/sec and the related separation forces were recorded on the force sensors. Different gap sizes (0.1-0.5 mm) were tested. The experiment result is shown in FIG. 6. It can be seen that:
(1) The separation force is smaller than the related cases with solidified resin;
(2) The separation force decreases with a larger gap size between the part and the PDMS film;
(3) The separation force can only be neglected until the gap size is larger than 0.5 mm.
The experiment results indicate that the suction force between the cured layer and the PDMS film may be large during the pulling-up process. Such a large force on the cured layer may cause the building process to fail if the bonding force between the current layer and previous layers is smaller than the suction force. In addition, after building multiple layers, such forces on the PDMS film may lead to produce cracks in the film due to material fatigue caused by the cyclic loading.